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Culture Equipment

When microbiologists want to identify microbes in a sample or study microbes in-depth, they often try to culture, or grow, the microbial cells in their labs. The scientists can then manipulate the cells or their environments to see what effects these changes have on the organisms.

These are the standard vessels in which microbial cells are grown. Petri plates are clear glass or acrylic dishes with lids that fit together like the two halves of a pillbox. Nutrients in either solid or liquid form can be put in them. Flasks are glass or acrylic bottle-like containers that can hold nutrients in liquid form.

Microbes require nutrients to grow. These are supplied by either solid or liquid culture media. The standard solid medium is nutrient agar, a gelatinous substance derived from seaweed. The basic liquid medium is nutrient broth, typically a mix of water, meat extract peptone, and sodium chloride.

Some microorganisms are more finicky than others and require media enriched with growth-promoting ingredients such as animal blood, glucose or egg. Examples of commonly used enriched media are blood agar, chocolate agar, and Loeffler medium.

Microbiologists also sometimes use special media called differential media that contain various chemicals designed to distinguish microbes by the appearances their colonies take on as they grow. For example, MacConkey agar has a stain called neutral red added and contains lactose (the sugar in milk). Lactic acid turns on the neutral red. Therefore, colonies formed by bacteria that ferment lactose into lactic acid will turn red and stand out from the pale colonies of non-lactose-fermenting bacteria.

When microbiologists want to inhibit the growth of certain microbes while encouraging the growth of others, they use special media called selective media. These media contain elements that inhibit the growth of some kinds of microbes and not others.

These are the tools microbiologists use to transfer microbial cells from one place to another. Pipettes combine the workings of a straw and a syringe in one instrument. Scientists use them to draw up liquids from one vial,

flask, or other container and squirt them in measured amounts into another flask, test tube, or other vessel.

Microbiologists use inoculating loops or swabs principally to inoculate petri plates with microbial cells. Loops are thin pieces of metal shaped into a small loop at one end.

To inoculate a plate, a microbiologist would first sterilize the tip of the swab or loop. Then he would dip or rub the loop over a contaminated surface or in a prepared sample. He then brushes the tip of the swab or loop across the nutrient agar in a plate in a series of zig-zag patterns. The zig-zagging causes microbial cells to be laid down in smaller clusters, allowing distinct and separate colonies to grow.